Sitting on a piece of land roughly the size of a football field in a former freight yard in Heidelberg, Germany, a new 162-unit apartment complex called Heidelberg Village is part of the Bahnstadt District, which will be the largest passive house development in the world.

To meet the exacting "Passivhaus" standard, buildings can only use a tiny amount of energy for heating and cooling. Even with cold German winters, the complex will never use more than 15 kilowatt-hours of energy for heating per square meter in a year; a "normal" building might use 100 to 300 kWh.

The scale of the development actually made it easier to save energy. "The reason is the volume to surface ratio," says Wolfgang Frey, head of Frey Architekten, the sustainable architecture firm that designed the complex. The buildings—one five stories high, and the other ranging from five to eight stories—are plastered with energy-producing solar panels on the facades, not just on the roof.

The placement of the panels and the buildings also helps save energy. "Because we are able to produce shade with the solar panels, it has a dual purpose: energy production all the time and cooling in the summertime," he says. "Because the dwellings are close together, we lose less heat and produce more energy by using every surface we can to generate it."

The design is as airtight as possible, but also includes windows that can open, something that used to be rare in buildings aiming to be ultra-efficient.

"About a decade ago it was common for passive houses to be built without openable windows," Frey says. "How ridiculous! First of all, it was a stupid idea to dictate to people that they are not allowed to open the windows. But it’s also is a stupid idea to use technical devices for air exchange throughout the entire year. What a waste of energy."

In the new apartments, residents will have the choice to open their windows or push a button to activate a ventilator. Air sensors also automatically detect the air quality and activate the air exchange if needed.

Outside, the buildings are covered in plants, both to help cool the apartments in the summertime and to improve the air quality. The paint on the walls also helps clear the air: titanium added to the wall color turns smog into harmless nitrates and oxygen.

The apartments were designed to house a diverse group of people: students, families, and elderly or disabled people who will have access to on-site health care. As the building is under construction, the architects have been inviting potential residents to come have lunch with construction workers. The goal is to give residents a sense of ownership in the buildings and inspire long-term responsibility for taking care of it.

Unlike some passive house apartment buildings—often criticized as boring boxes—the architects aimed for a liveable design. "Our buildings simulate the feeling of living beneath old trees. You have the feeling of being protected by the massive structure and the overhanging boughs, but you also have the freedom of open space beneath the forest to the open field," says Frey.

The complex will be completed in late 2017.

Correction: The headline and first paragraph of this article originally cited Heidelberg village as the largest passive-house apartment complex in the world. It's not on its own, but it is part of the Bahnstadt District, which will be the largest passive-house complex in the world when it's complete (it also includes retail). We regret the error.

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